Click here to see a bigger annotated picture of Auto-Tune's interface.

The main thing is to set the Retune speed (1) to zero. Once you've done that your vocal will be snapped to the nearest semitone practically instantaneously. You can also pick the key and scale you want to use (2), though this won't have any effect until you pick major or minor from the pitch correction panel (3).

Picking major or minor will take notes not in the scale and snap them to the nearest note in the chosen scale. You can manually change these settings, and for a really messed up effect you can take away all but a few pitches.